Diode and method for manufacturing the same

ABSTRACT

A diode is provided which includes a first-conductivity-type cathode layer, a first-conductivity-type drift layer placed on the cathode region and having a lower concentration than the cathode layer, a generally ring-like second-conductivity-type ring region formed in the drift layer, second-conductivity-type anode region formed in the drift layer located inside the ring region, a cathode electrode formed in contact with the cathode layer, and an anode electrode formed in contact with the anode region, wherein the lowest resistivity of the second-conductivity-type anode region is at least {fraction (1/100)} of the resistivity of the drift layer, and the thickness of the anode region is smaller than the diffusion depth of the ring region.

FIELD OF THE INVENTION

[0001] The present invention relates to a pn junction diode having a pnjunction, a Schottky diode having a Schottky junction, and a compositediode having both of the pn junction and Schottky junction, each ofthese diodes having a rectifying function.

BACKGROUND OF THE INVENTION

[0002] Diodes having a rectifying function are the most fundamentalsemiconductor elements or components, and various types of diodes areknown which have different junction structures.

[0003]FIG. 37 is a cross-sectional view showing a pn junction diode 101having a basic planar-type pn junction. To provide the diode 101, ahigh-concentration n⁺ cathode layer 1 is formed on one of oppositesurfaces of a low-concentration n drift layer 2, and a p anode region 3is formed in a surface layer at the other surface of the n drift layer2. Cathode electrode 4 and anode electrode 5 are formed in contact withthe surfaces of the n⁺ cathode layer 1 and p anode region 3,respectively. The diode 101 further includes an oxide film 6 that coversthe surface of the pn junction, and a protective film 7 in the form of anitride film. A p-type peripheral region 8 is formed in a peripheralportion of the pn junction diode 101, and a peripheral electrode 11 isprovided on the surface of the peripheral region 9, to extend over apart of the oxide film 6.

[0004] The n drift layer 2 is laminated by epitaxial growth on the n⁺cathode layer 1 as a substrate. For example, the impurity concentrationsof the n⁺ cathode layer 1 and n drift layer 2 are 1×10¹⁹ cm⁻³, and1×10¹⁵ cm⁻³, respectively, and the thicknesses of these layers 1, 2 are450 μm and 10 μm, respectively. The p anode region 3 is formed byimplanting p-type impurities, such as boron ions, using the oxide film 6as a mask, and thermally diffusing the implanted ions. The p anoderegion 3 thus formed has a surface impurity concentration of 1×10¹⁹cm⁻³, and a diffusion depth of 3 μm.

[0005] The graph of FIG. 38 shows a profile of the resistivity measuredalong a cross section of the pn junction diode 101 of FIG. 37. In FIG.38, the vertical axis indicates the thickness as measured from thesurface of the semiconductor substrate including the n cathode layer 1and n drift layer 2, and the horizontal axis indicates the resistivityplotted on a logarithmic scale. As shown in the cross section, the diode101 includes the p anode region 3 having a thickness of 3 μm as measuredfrom the surface of the semiconductor substrate, n drift layer 2 havinga thickness of about 60 μm, and the n cathode layer 1 having a lowresistivity, which is formed under the n drift layer 2. Generally, theresistivity of a portion of the surface of the p anode region 3 whichhas the lowest resistance is about 0.01 Ω·m.

[0006]FIG. 39 is a cross-sectional view of a pn junction diode 102 whichis a slightly modified example of the planar-type diode of FIG. 37. Asin the pn junction diode 101 of FIG. 37, a high-concentration n⁺ cathodelayer 1 and a low-concentration n drift layer 2 constitute asemiconductor substrate, and a p anode region 3 is formed in a surfacelayer of the n drift layer 2 of the semiconductor substrate. The pnjunction diode 102 is different from the diode 101 of FIG. 37 in that ap ring region 12 having a ring-like shape and a large diffusion depth isformed at the outer periphery of the p anode region 3. While breakdownof the pn junction diode of FIG. 37 is likely to occur in the vicinityof the periphery of the p anode region 3, the p ring region 12 having alarger diffusion depth than the p anode region 3 is formed in the diodeof FIG. 39, so as to decrease the gradient of the impurityconcentration, thereby to prevent occurrence of the breakdown at aroundthe p anode region 3. As a result, the breakdown occurs uniformlythroughout the p anode region 3.

[0007]FIG. 40 is a cross-sectional view of a pn junction diode 103 inwhich p high-concentration regions 13 having a high surface impurityconcentration and a large diffusion depth are formed between p anoderegions 3 having a low surface impurity concentration and a smalldiffusion depth, as disclosed in Shimizu et al., IEEE Trans. on ElectronDevices ED-31, (1984) p. 1314). When rated current is applied to thediode, the current flows through the p anode regions 3, and thereforethe diode exhibits an excellent reverse recovery characteristic. In thereverse bias situation, a depletion layer spreads out from the phigh-concentration regions 13, and thus the diode shows a high breakdownvoltage. The p high-concentration regions 13 may also serve as the pring region 12 as described above.

[0008]FIG. 41 is a cross-sectional view of a Schottky diode 104 having abasic Schottky junction. To form the diode 104, a Schottky electrode 15made of a metal, such as molybdenum, which provides a high Schottkybarrier, is formed on a surface of a low-concentration n drift layer 2.A cathode electrode 4 is provided on the rear surface of a n⁺ cathodelayer 1. A p ring region 12 is formed in a surface layer of the n driftlayer 2 so as to surround a contact portion of the Schottky electrode15. With the p ring region 12 thus provided, an electric field isprevented from concentrating at the edge of the Schottky electrode 15,and the breakdown voltage of the resulting diode can be increased.

[0009] The n drift layer 2 is laminated by epitaxial growth on thehigh-concentration n⁺ cathode layer 1 serving as a substrate. Forexample, the n⁺ cathode layer 1 has a resistivity of 0.004 Ω·cm, and athickness of 350 μm, and the n drift layer 2 has a resistivity of 0.90Ω·cm, and a thickness of 7 μm.

[0010] The graph of FIG. 38 also shows a profile of the resistivitymeasured along a cross section of the Schottky diode 104 of FIG. 41. Thevertical axis indicates the depth as measured from the surface of thesemiconductor substrate comprising the n⁺ cathode layer and n driftlayer 2, and the vertical axis indicates the resistivity plotted on alogarithmic scale. In the case of a Schottky diode having a breakdownvoltage of 60V, for example, the n drift layer 2 having a resistivity of0.9 Ω·cm extends from the surface of the semiconductor substrate to adepth of about 7 μm, and the n⁺ cathode layer 1 having a resistivity of0.0040Ω·cm is formed under the n drift layer 2.

[0011]FIG. 42 is a cross-sectional view showing a Schottky diode 105 asa slightly modified example of the Schottky diode 104 of FIG. 41. In thediode 105, trenches 16 are formed in a surface layer of the n driftlayer 2, and a Schottky electrode 15 made of molybdenum, for example, isformed on the surface of the n drift layer 2 and the inner walls of thetrenches 16. With the trenches 16 thus provided, a contact area of theSchottky electrode 15 is increased, thereby to increase the currentcapacitance.

[0012]FIG. 43 is a cross-sectional view of a composite diode 106 havinga pn junction and a Schottky junction. In the composite diode 106, arelatively wide p ring region 12 is formed in a surface layer of an ndrift layer 2 so as to surround a contact portion of a Schottkyelectrode 15, such that the Schottky electrode 15 is in contact with thesurface of the p ring region 12 as well as the n drift layer 2, asdisclosed in Zettler, R. A. et al.: IEEE Trans. on Electron DevicesED-16, (1969) p. 58. In this case, the p ring region 12 provides a panode region 3 of a pn junction diode. Thus, the composite diode, inwhich the pn junction and Schottky junction are combined, provides a lowforward voltage when it is forward biased, a high breakdown voltage, andan effect of reducing noise.

[0013]FIG. 44 is a cross-sectional view of a composite diode 107 whichis a modified example of the composite diode of FIG. 43. In thisexample, not only the p ring region 12 is formed at the periphery of then drift layer 2 that contacts with the Schottky electrode 15, but also panode regions 3 are formed inside the p ring region 12. The Schottkyelectrode 15 is formed in contact with both exposed portions of the ndrift layer 2 and the surfaces of the p anode regions 3, as disclosed inJapanese Patent No. 59-35183. The exposed portions of the n drift layer2 between the p anode regions 3 have a small width, and a depletionlayer spreads out from the p anode regions 3 when a reverse bias isapplied, assuring a reduced leakage current.

[0014]FIG. 45 is a cross-sectional view of a composite diode 108 whichis a slightly modified example of the composite diode 107 of FIG. 44. Inthis example, a p anode region 3 is formed in a surface layer of the ndrift layer 2, and trenches 16 having a larger depth than the p anoderegion 3 are formed. Further, a Schottky electrode 15 made ofmolybdenum, for example, is formed in contact with the surface of the panode region 3 and the inner walls of the trenches 16. In this case,too, the provision of the trenches 15 leads to an increase in thecontact area of the Schottky electrode 15, and an increase in thecurrent capacitance.

[0015]FIG. 46 is a cross-sectional view of a composite diode 109 whichis a slightly modified example of the composite diode 108 of FIG. 45. Inthis example, trenches 16 are formed in a surface layer of an n driftlayer 2, and p anode regions 3 are formed along the inner faces of thetrenches 16. Schottky electrode 15 is formed in contact with both asurface layer of the n drift layer 2 where the trenches 16 are notformed, and the surfaces of the p anode regions 3 formed along the innerwalls of the trenches (see Kunori, S et al.: Proc. of 1992 Intern. Symp.on Power Semicond. Devices and ICs, Tokyo, (1992) p.69). By providingthe trenches 16, and forming the p anode regions 3 on the inner walls ofthe trenches 16, the leakage current can be reduced in the reverse biassituation.

[0016] In the pn junction diodes of FIGS. 37, 39, 40, lifetime killersfor accelerating recombination of accumulated carriers are introduced bydiffusion of Au or Pt, or irradiation of electron beams, so as toincrease the switching speed. However, the introduction of the lifetimekillers induces or causes an increase of leakage current. Namely, theleakage current I_(R) increases if the reverse recovery time t_(rr) isshortened by introducing a lot of lifetime killers, and the reverserecovery time t_(rr) is increased if the leakage current I_(R) isreduced. Thus, there is a trade-off relationship between the reverserecovery time t_(rr) and the leakage current I_(R). As another problem,the reverse recovery waveform shows hard recovery if a lot of lifetimekillers are introduced.

[0017] In the Schottky diode of FIGS. 41 and 42, there is a trade-offrelationship between the ON-state voltage V_(F) and the reverse leakagecurrent I_(R). The ON-state voltage V_(F) may be reduced by using ametal having a small barrier height, or lowering the resistance of the nregion. In this case, however, the leakage current I_(R) is undesirablyincreased in the reverse bias situation. If a metal having a largebarrier height is used, or the resistance of the n region is increased,the leakage current I_(R) is reduced, but the ON-state voltage V_(F) isincreased. Thus, there is a trade-off relationship between the ON-statevoltage V_(F) and the reverse leakage current I_(R).

[0018] The composite diodes of FIG. 43 through FIG. 46 have a parallelstructure of pn junction diode and Schottky diode, and make use ofadvantages of the respective types of diodes. These composite diodes,however, inherit disadvantages or problems of the pn junction diode andSchottky diodes.

[0019] Furthermore, conventional diodes generally have a low ability towithstand avalanche breakdown. In particular, the pn junction has acertain radius of curvature at around a corner portion of the p anoderegion or p ring region, and therefore the ability to withstandavalanche breakdown is lowered due to concentration of an electric fieldon the corner portion, as compared with that of the planar pn junction.Thus, the conventional diodes tend to break down due to concentration ofcurrent that may result in avalanche breakdown.

SUMMARY OF THE INVENTION

[0020] It is therefore an object of the present invention to provide adiode having a high switching speed, reduced leakage current, reducedforward voltage, and high ability to withstand avalanche breakdown. Itis another object to provide a method for manufacturing such a diode.

[0021] To accomplish the above object, the present invention provides adiode comprising: a first-conductivity-type cathode layer as a firstregion; a first-conductivity-type drift layer as a second region placedon the cathode layer and having a lower impurity concentration than thecathode layer; a generally ring-like second-conductivity-type ringregion as a third region formed in a surface layer of thefirst-conductivity-type drift layer; a second-conductivity-type anoderegion as a fourth region formed in a surface layer of thefirst-conductivity-type drift region located inside the ring region; acathode electrode as a first main electrode formed in contact with thesecond-conductivity-type cathode layer; and an anode electrode as asecond main electrode formed in contact with thesecond-conductivity-type anode region, wherein a portion of thesecond-conductivity-type anode region having the lowest resistance has aresistivity which is at least {fraction (1/100)} of that of thefirst-conductivity-type drift layer, and the second-conductivity-typeanode region has a thickness which is smaller than a diffusion depth ofthe second-conductivity-type ring region.

[0022] With the diode constructed as described above, the amount ofminority carriers injected into the first-conductivity-type drift layeris significantly reduced, and the carriers accumulated in this layer areaccordingly reduced, whereby the reverse recovery time is shortened.

[0023] Preferably, the lowest resistivity of thesecond-conductivity-type anode region is in a range of 0.3 to 30 timesthe resistivity of the first-conductivity-type drift layer. In thiscase, the accumulated carriers are further reduced, resulting in afurther shortened reverse recovery time. Also, lifetime killers are notnecessary or only a small amount of lifetime killers need to beintroduced so as to control the reverse recovery time to a given value.Accordingly, the leakage current arising upon application of a reversebias is also considerably reduced.

[0024] In the diode as described above, the second-conductivity-typeanode region preferably has a diffusion depth in a range of 0.01 to 0.5μm. In this case, the total amount of impurities is reduced, which iseffective to reduce the amount of minority carriers injected into thefirst-conductivity-type drift layer. If the thickness of thehigh-resistance second conductivity-type anode region is larger than 0.5μm, the forward loss in this layer is increased.

[0025] In a method for manufacturing the diode as described above, thesecond-conductivity-type anode region is formed by implantingsecond-conductivity-type ions in a dose amount of 1×10¹⁰ to 1×10¹² cm⁻²,and conducting heat treatment.

[0026] According to the manufacturing method as described above, theresistivity of the second-conductivity-type anode region can be easilycontrolled to be {fraction (1/100)} or higher than that of thefirst-conductivity-type drift layer, and its thickness can be easilycontrolled to 0.5 μm or smaller.

[0027] If the heat treatment for forming the second-conductivity-typeanode region is conducted at a temperature in a range of 300 to 600° C.,the activation rate of impurities provided by ion implantation can besuitably controlled, without significantly changing the junctionstructure. If the temperature of the heat treatment is less than 300° C.or higher than 600° C., the diffusion depth is undesirably increased.

[0028] According to another aspect of the present invention, there isprovided a diode comprising: a first-conductivity-type cathode layerhaving a first impurity concentration; a first-conductivity-type driftlayer placed on the cathode layer and having a lower impurityconcentration than the cathode layer, the cathode layer and drift layerconstituting a semiconductor substrate; a generally ring-likesecond-conductivity-type third ring region formed in a surface layer ofthe first-conductivity-type drift layer; a cathode electrode formed incontact with the cathode layer; a Schottky electrode as a third mainelectrode which contacts with a surface of the semiconductor substrateinside the second-conductivity-type ring region, so as to form aSchottky junction; and a first-conductivity-type low-concentrationregion as a fifth region formed in a surface layer of the drift layerlocated inside the second-conductivity-type ring region, thelow-concentration region having a higher resistivity than thefirst-conductivity-type drift layer, and having a thickness that issmaller than a diffusion depth of the second-conductivity-type ringregion.

[0029] In the diode as described just above, the surface of thesemiconductor substrate that contacts with the Schottky electrodeprovides a first-conductivity-type high-resistance region having a highresistivity, which contributes to reduction of the leakage currentduring application of a reverse bias.

[0030] In particular, if the thickness of the first-conductivity-typehigh-resistance region is in a range of 0.01 to 3.0 μm, more preferably,in a range of 0.01 to 0.5 μm, the high-resistance region that contactswith Schottky electrode satisfactorily yields the above effect. If thisthickness exceeds 0.5 μm, the forward loss is increased in this layer.

[0031] Preferably, the highest resistivity of thefirst-conductivity-type high-resistance region is in a range of 1.2 to12 times that of the first-conductivity-type drift layer. If the highestresistivity is less than 1.2 times the resistivity of the drift layer,the effect of reducing the leakage current is insufficient. If itexceeds 12 times, on the other hand, the forward loss in this layer isincreased to a level that is not negligible.

[0032] In a method for manufacturing the diode as described above, thefirst-conductivity-type high-resistance region is formed by implantingsecond-conductivity-type ions in a dose amount in a range of 1×10¹⁰ to1×10¹³ cm⁻², preferably, in a range of 1×10¹⁰ to 1×10¹² cm⁻², andconducting heat treatment.

[0033] The first-conductivity-type high-resistance region may also beformed by epitaxial growth.

[0034] In the manufacturing method as described above, the resistivityof the first-conductivity-type high-resistance region can be easilycontrolled to be 1.2 to 12 times that of the first conductivity-typedrift layer, and its diffusion depth can be easily controlled to a rangeof 0.1 to 0.5 μm.

[0035] If the heat treatment for forming the first-conductivity-typehigh-resistance region is conducted at a temperature in a range of 300to 600° C., the activation rate of impurities provided by ionimplantation can be suitably controlled, without significantly changingthe junction structure. If the temperature of the heat treatment is lessthan 300° C., the impurities are not sufficiently activated. If thetemperature exceeds 600° C., the diffusion depth is increased.

[0036] According to a further aspect of the present invention, there isprovided a diode comprising: a first-conductivity-type cathode layerhaving a first impurity concentration, a first-conductivity-type driftlayer placed in the cathode layer and having a second impurityconcentration that is lower than the first impurity concentration; asecond-conductivity-type anode region formed in a surface layer of thefirst-conductivity-type drift layer; a first-conductivity-type embeddedregion as a sixth region formed in contact with thefirst-conductivity-type cathode layer located below thesecond-conductivity-type anode region, the embedded region having alower resistivity than the drift layer; a cathode electrode formed incontact with the cathode layer; and an anode electrode formed in contactwith the anode region; wherein the first-conductivity-type embeddedregion is formed only inside an area defined by a vertical projection ofthe second-conductivity-type anode region.

[0037] In the diode constructed as described above, the thickness of theportion of the first-conductivity-type drift layer interposed betweenthe second-conductivity-type anode region and thefirst-conductivity-type embedded region is reduced. When a reverse biasis applied to this diode, therefore, current that may cause avalanchebreakdown flows uniformly through a relatively wide region where theembedded region is formed.

[0038] In a diode in which a generally ring-likesecond-conductivity-type ring region is formed in a surface layer of afirst-conductivity-type drift layer, and a second conductivity-typeanode region is formed in a surface layer of the first-conductivity-typedrift layer located inside the second-conductivity-type ring region, afirst-conductivity-type embedded region is formed only inside an areadefined by a vertical projection of the second-conductivity-type ringregion, and the thickness of the first-conductivity-type drift layerlocated on the embedded region is smaller than the thickness of thedrift layer located under the ring region.

[0039] In the diode as described just above, the thickness of theportion of the first-conductivity-type drift layer that is interposedbetween the second-conductivity-type anode region and thefirst-conductivity-type embedded region is smaller than that of thedrift layer under the second-conductivity-type ring region. When areverse bias is applied to this diode, therefore, current that may causethe avalanche breakdown flows uniformly through a wide region where thefirst-conductivity-type embedded region is formed.

[0040] In a diode in which a generally ring-likesecond-conductivity-type ring region is formed in a surface layer of afirst-conductivity-type drift layer, and a Schottky electrode whichcontacts with a surface of a semiconductor substrate inside thesecond-conductivity-type ring region so as to form a Schottky junction,the first-conductivity-type embedded region is formed only inside anarea defined by a vertical projection of the second-conductivity-typering region, for the same reason as described above, and the thicknessof the first-conductivity-type drift layer on thefirst-conductivity-type embedded region is preferably smaller than thatof the drift layer located under the second-conductivity-type ringregion.

[0041] In one preferred form of the diode as described above, thefirst-conductivity-type embedded region is formed inside an area definedby a vertical projection of the second-conductivity-type ring region,with a spacing of 5 μm or larger between the periphery of the embeddedregion and the vertical projection. With this arrangement, current whichmay cause avalanche breakdown flows uniformly toward thefirst-conductivity-type embedded region, without affecting thesecond-conductivity-type ring region.

[0042] In a diode including trenches formed in a surface layer of afirst-conductivity-type drift layer, a first-conductivity-type embeddedregion formed in contact with a portion of a first-conductivity-typecathode layer located below the trenches, and a second conductivity-typeanode region formed along a surface of the drift layer and inner facesof the trenches, the first-conductivity-type embedded region is formedonly inside an area of a vertical projection of the outermost trenches.

[0043] In a diode including trenches formed in a surface layer of afirst-conductivity-type drift layer, a first-conductivity-type embeddedregion formed in contact with a portion of a first-conductivity-typecathode layer located below the trenches, and a Schottky electrode whichcontacts with at least a part of the surface of thefirst-conductivity-type drift layer and inner faces of the trenches, thefirst-conductivity-type embedded region is formed only inside an area ofa vertical projection defined by the outermost trenches.

[0044] With the diode constructed as described above, the electric fieldis prevented from concentrating at bottom parts of the outermosttrenches, and breakdown occurs uniformly between bottom parts of innertrenches and the first-conductivity-type embedded region.

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] The invention will be described in greater detail with referenceto preferred embodiments thereof and the accompanying drawings, wherein:

[0046]FIG. 1 is a cross-sectional view of a pn junction diode accordingto the first embodiment of the present invention;

[0047]FIG. 2 is a graph showing a profile of the resistivity of the pnjunction diode of the first embodiment;

[0048]FIG. 3 is a graph showing a profile of the resistivity of a pnjunction diode according to the second embodiment of the invention;

[0049]FIG. 4(a) through FIG. 4(d) are cross-sectional views showing mainprocess steps of a method for manufacturing the pn junction diode of thefirst embodiment;

[0050]FIG. 5 is a cross-sectional view of a pn junction diode accordingto the third embodiment of the present invention;

[0051]FIG. 6 is a cross-sectional view of a pn junction diode accordingto the fourth embodiment of the invention;

[0052]FIG. 7 is a cross-sectional view of a Schottky diode according tothe fifth embodiment of the invention;

[0053]FIG. 8 is a graph showing a profile of the resistivity of theSchottky diode of the fifth embodiment.

[0054]FIG. 9(a) through FIG. 9(d) are cross-sectional views showing mainprocess steps of a method for manufacturing the pn junction diode of thefifth embodiment;

[0055]FIG. 10 is a cross-sectional view of a Schottky diode according tothe sixth embodiment of the invention;

[0056]FIG. 11 is a graph showing a profile of the resistivity of theSchottky diode of the sixth embodiment of the invention;

[0057]FIG. 12(a) through FIG. 12(d) are cross-sectional views showingmain process steps of a method for manufacturing the pn junction diodeof the sixth embodiment;

[0058]FIG. 13 is a cross-sectional view of a Schottky diode according tothe seventh embodiment of the present invention;

[0059]FIG. 14 is a cross-sectional view of a Schottky diode according tothe eighth embodiment of the invention;

[0060]FIG. 15 is a cross-sectional view of a Schottky diode according tothe ninth embodiment of the invention;

[0061]FIG. 16 is a cross-sectional view of a Schottky diode according tothe tenth embodiment of the invention;

[0062]FIG. 17 is a cross-sectional view of a pn junction diode accordingto the eleventh embodiment of the invention;

[0063]FIG. 18 is a graph showing a profile of the resistivity of the pnjunction diode of the eleventh embodiment;

[0064]FIG. 19(a) through FIG. 19(e) are cross-sectional views showingmain process steps of a method for manufacturing the pn junction diodeof the eleventh embodiment;

[0065]FIG. 20 is a view showing a mask used for forming an n⁺ embeddedlayer of the pn junction diode of the eleventh embodiment;

[0066]FIG. 21(a) through FIG. 21(d) are cross-sectional views showingmain process steps of a method for forming a pn junction diode accordingto the twelfth embodiment of the invention;

[0067]FIG. 22 is a cross-sectional view of a pn junction diode accordingto the thirteenth embodiment of the present invention;

[0068]FIG. 23 is a cross-sectional view of a pn junction diode accordingto the fourteenth embodiment of the invention;

[0069]FIG. 24 is a cross-sectional view of a pn junction diode accordingto the fifteenth embodiment of the invention;

[0070]FIG. 25 is a cross-sectional view of a pn junction diode accordingto the sixteenth embodiment of the invention;

[0071]FIG. 26 is a cross-sectional view of a pn junction diode accordingto the seventeenth embodiment of the invention;

[0072]FIG. 27 is a cross-sectional view of a Schottky diode according tothe eighteenth embodiment of the invention;

[0073]FIG. 28 is a cross-sectional view of a Schottky diode according tothe nineteenth embodiment of the invention;

[0074]FIG. 29 is a cross-sectional view of a Schottky diode according tothe twentieth embodiment of the invention;

[0075]FIG. 30 is a cross-sectional view of a Schottky diode according tothe twenty-first embodiment of the invention;

[0076]FIG. 31 is a cross-sectional view of a Schottky diode according tothe twenty-second embodiment of the invention;

[0077]FIG. 32 is a cross-sectional view of a Schottky diode according tothe twenty-third embodiment of the invention;

[0078]FIG. 33 is cross-sectional view of a composite diode according tothe twenty-fourth embodiment of the invention;

[0079]FIG. 34 is a cross-sectional view of a composite diode accordingto the twenty-fifth embodiment of the invention;

[0080]FIG. 35 is a cross-sectional view of a composite diode accordingto the twenty-sixth embodiment of the invention;

[0081]FIG. 36 is a cross-sectional view of a composite diode accordingto the twenty-seventh embodiment of the invention;

[0082]FIG. 37 is a cross-sectional view showing a known example of pnjunction diode;

[0083]FIG. 38 is a graph showing a profile of the resistivity of each ofknown pn junction diode and Schottky diode;

[0084]FIG. 39 is a cross-sectional view of another known example of pnjunction diode;

[0085]FIG. 40 is a cross-sectional view of a further known example of pnjunction diode;

[0086]FIG. 41 is a cross-sectional view of a known example of Schottkydiode;

[0087]FIG. 42 is a cross-sectional view of another known example ofSchottky diode;

[0088]FIG. 43 is a cross-sectional view of a known example of compositediode;

[0089]FIG. 44 is a cross-sectional view of another known example ofcomposite diode;

[0090]FIG. 45 is a cross-sectional view of a further known example ofcomposite diode; and

[0091]FIG. 46 is a cross-sectional view of a still another known exampleof composite diode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0092] Some embodiments of the present invention will be described withreference to the drawings. In the following embodiments, regions andlayers to which “n” and “p” are prefixed represent those havingelectrons and holes as majority carriers, respectively. While n type isregarded as the first conductivity type and p type is regarded as thesecond conductivity type in these embodiments, these conductivity typemay be reversed.

[0093] First Embodiment

[0094]FIG. 1 is a cross-sectional view of a pn junction diode 201 havinga pn junction according to the first embodiment of the presentinvention. The diode 203 is different from the known pn junction diodeof FIG. 37 in that a p⁻ anode region 3 a having a very small depth isformed in a surface layer of a low-concentration n drift layer 2. Anodeelectrode 5 and cathode electrode 4, which are in contact with thesurface of the p anode region 3 a and the rear surface of ahigh-concentration n⁺ cathode layer 1 formed on the other side of the ndrift layer 2 opposite to the p⁻ anode region 3 a, respectively, andoxide film 6 that covers the surface of the pn junction, protective filmin the form of a nitride film, and others are similar to those of theknown pn junction diode. A p⁻ type peripheral region 8 is formed in aperipheral portion of the pn junction diode 201, and a peripheralelectrode 11 is formed on the surface of the peripheral region 8, toextend over a part of the oxide film 6. Depending upon the case, an ntype peripheral region may be provided in place of the p peripheralregion 8.

[0095]FIG. 4(a) through FIG. 4(d) are cross-sectional views showing mainprocess steps of a method for manufacturing the pn junction diode 201 ofthe first embodiment. The n drift layer 2 is laminated by epitaxialgrowth on the n⁺ cathode layer 1 as a substrate, which is doped witharsenic, and has a resistivity of 0.0040 Ω·cm and a thickness of 350 μm.The n drift layer 2 is doped with phosphorous, and has a resistivity of40 Ω·m and a thickness of 60 μm. The oxide film 6 is formed by thermallyoxidizing the epitaxial wafer thus formed, and patterned byphotolithography to provide a mask. Then, a thin oxide film is formed,and the p ring region 12 and p peripheral region 8 are formed in asurface layer of the n drift layer 2, through implantation of boron ionsand subsequent thermal diffusion, as shown in FIG. 4(a). Theacceleration voltage for ion implantation is 45 keV, and the dose amountis 5×10¹³ cm⁻². After the ion implantation, annealing is conducted at1150° C. for 200 min. The surface impurity concentration of the p ringregion 12 and p peripheral region 8 is about 2×10¹⁷ cm⁻³, and thediffusion depth is 3 μm. For lifetime control, gold or platinum isdiffused at 700 to 800° C.

[0096] Subsequently, the oxide film 6 located inside the p ring region12 is removed by photolithography, and the p⁻ anode region 3 a is formedby implantation of boron ions and thermal diffusion, as shown in FIG.4(b). The acceleration voltage for ion implantation is 45 keV, and thedose amount is 1×10¹² cm⁻². After the ion implantation, annealing isconducted at 450° C. for 30 min. The surface impurity concentration ofthe p⁻ anode region 3 a is 5×10¹⁵ cm⁻³, and the depth of junctionbetween the p⁻ anode region 3 a and the n drift layer 2 is as small asabout 0.3 μm because the annealing temperature is low, and thermaldiffusion hardly takes place. The activation rate of the implantedimpurities is as low as about 0.1 because of the low annealingtemperature.

[0097] A 5 μm-thickness aluminum alloy is deposited by sputtering on thesurface of the structure shown in FIG. 4(b), and patterned byphotolithography to thus form the anode electrode 5 and peripheralelectrode 11, as shown in FIG. 4(c).

[0098] Subsequently, a nitride film is deposited by a plasma CVD method,and patterned by photolithography, to provide the protective film 7, andthree layers of titanum, nickel and gold are vapor-deposited on the rearsurface of the n⁺ cathode layer 1, to thus form the cathode electrode 4,as shown in FIG. 4(d).

[0099] The graph of FIG. 2 shows a profile of the resistivity measuredalong a cross section of the pn junction diode 201 of FIG. 1. Thehorizontal axis indicates the depth as measured from the surface of thesemiconductor substrate comprising the n drift layer 2 and n⁺ cathodelayer 1, and the vertical axis indicates the resistivity as plotted on alogarithmic scale. The p⁻ anode region 3 a having a depth of 0.3 μm liesat the surface of the semiconductor substrate, and the n drift layer 2having a thickness of about 60 μm and the n⁺ cathode layer 1 having alow resistivity are present in this order under the p anode region 3 a.A portion of the surface of the p anode region 3 having the lowestresistance has a resistivity of about 0.80 Ω·cm, which is larger than0.4 Ω·cm which is one-hundredth of the resistivity of the n drift layer2.

[0100] The total amount of impurities contained in the p⁻ anode region 3a of the pn junction diode 201 of the first embodiment is smaller byabout three orders of magnitude than that of the known pn junctiondiode. Therefore, the amount of minority carriers injected from the p⁻anode region 3 a into the n drift layer 2 is remarkably reduced, and thereverse recovery time t_(rr) is shortened.

[0101] In order to control the reverse recovery time t_(rr) to a givenvalue, on the other hand, the amount of lifetime killers may be aboutone-tenth of that of the known diode. If the reverse recovery timet_(rr) is to be controlled to 70 ns, for example, platinum that provideslifetime killers may be diffused at a temperature which is 70° C. lowerthan the known diffusion temperature. Accordingly, the leakage currentI_(R) is also reduced to about one-tenth of that of the known diode. Inaddition, the reverse recovery waveform exhibits soft recovery owing toincreased lifetime.

[0102] Second Embodiment

[0103] The graph of FIG. 3 shows a profile of the resistivity measuredalong a cross section of a pn diode 202 of the second embodiment of thepresent invention. In FIG. 3, the horizontal axis indicates the depth asmeasured from the surface of the semiconductor substrate, and thevertical axis indicates the resistivity as plotted on a logarithmicscale. In the pn junction diode 202, the p⁻ anode region 3 a having adepth of 0.3 μm lies at the surface of the semiconductor substrate, andthe n drift layer 2 having a thickness of about 60 μm and the n⁺ cathodelayer 1 having a low resistivity are present in this order under the p⁻anode region 3 a. In this embodiment, a portion of the surface of the p⁻anode region 3 a having the lowest resistance has a resistivity of about40 Ω·cm, which is substantially equivalent to the resistivity of the ndrift layer 2.

[0104] The total amount of impurities contained in the p⁻ anode region 2of the pn junction diode 202 of the second embodiment is reduced byadditional two orders of magnitude than that of the pn junction diode201 of the first embodiment. Accordingly, a further reduced amount ofminority carriers are injected into the n drift layer 2, and the reverserecovery time t_(rr) is shortened.

[0105] In order to control the reverse recovery time t_(rr) to a givenvalue, the amount of lifetime killers may be about one-twentieth of thatof the known diode. Accordingly, the leakage current I_(R) is alsoreduced to about one-twentieth of that of the known diode. In addition,the reverse recovery waveform exhibits soft recovery owing to increasedlifetime

[0106] Third Embodiment

[0107]FIG. 5 is a cross-sectional view of a pn junction diode 203constructed according to the third embodiment of the present invention.To produce the diode 203, trenches 15 are formed in a surface layer ofan n drift layer 2, and a p⁻ anode region 3 a having a small diffusiondepth is formed at the surface of the n drift layer 2 and the innerfaces of the trenches 16, such that the total impurity amount in theregion 3 a is smaller by some orders of magnitude than that of the knowndiode, as in the first embodiment. The trenches 16 have a width of 1 μmand a depth of 3 μm, and are arranged at intervals of 2 μm.

[0108] In this embodiment, too, the amount of minority carriers injectedfrom the p⁻ anode region 3 a into the n drift layer 2 is considerablyreduced, and therefore the reverse recovery time t_(rr) is shortened.With the trenches 16 thus provided, the area of the pn junction and thecontact area of the anode electrode 5 are increased, with a result of anincrease in the current capacitance. The trenches 16, as viewed in theplane of the substrate, may be in various forms, such as stripes orlattice.

[0109] Fourth Embodiment

[0110]FIG. 6 is a cross-sectional view of a pn junction diode 204according to the fourth embodiment of the present invention. To producethe diode 204, a p anode region 3 having a large total amount ofimpurities is formed in a surface layer of the n drift layer 2, andtrenches 16 having a larger depth than the p anode region 3 are formedin the n drift layer 2 through the p anode region 3. Further, p⁻ anoderegions 3 a having a small diffusion depth are formed in the inner facesof portions of the trenches 16 that are deeper than the p anode region3, such that the total impurity amount of the p⁻ anode regions 3 a issmaller by some orders of magnitude than that of the known diode, as inthe first embodiment. In this embodiment, the p anode region 3 having alarge total amount of impurities serves to increase the amount ofinjection of the minority carriers, in order to reduce the ON-statevoltage in a high current region, while the trenches 16 are provided forincreasing the contact area between the p anode regions 3 a having asmall diffusion depth, and the anode electrode 5, thereby to increasethe current capacitance.

[0111] Fifth Embodiment

[0112]FIG. 7 is a cross-sectional view of a Schottky diode 205 having aSchottky barrier constructed according to the fifth embodiment of thepresent invention. The Schottky diode 205 is different from the knownSchottky diode of FIG. 39 in that an n⁻ high-resistance region 17 havinga considerably low impurity concentration is formed in a surface layerof an n drift layer 2, and a Schottky electrode 15 made of a metal thatforms a Schottky junction is formed in contact with the n⁻high-resistance region 17. The other portions of the diode 205, namely,cathode electrode 4 that contacts with the rear surface of the n⁺cathode layer 1, p ring region 12, oxide film 6 that covers the surfaceof the pn junction, protective film 7 in the form of a nitride film, andothers, are similar to those of the known Schottky diode. A p⁻ typeperipheral region 8 is formed in a peripheral portion of the Schottkydiode 205, and a peripheral electrode 11 is formed on the surface of theperipheral region 8, to extend over a part of the oxide film 6.Depending upon the case, the p-type peripheral region 8 may be replacedby an n-type peripheral region.

[0113]FIG. 9(a) through FIG. 9(d) are cross-sectional views showing mainprocess steps of a method for manufacturing the Schottky diode 205 ofthe fifth embodiment. The n drift layer 2 is laminated by epitaxialgrowth on the n⁺ cathode layer 1 as a substrate, which has a resistivityof 0.004 Ω·cm and a thickness of 350 μm. In the case where the Schottkydiode has a breakdown voltage of 60V, the n drift layer 2 has aresistivity of 0.55 Ω·m and a thickness of 7 μm. The oxide film 6 isformed by thermally oxidizing the epitaxial wafer thus formed, andpatterned by photolithography to provide a mask. Then, a thin oxide filmis formed, and the p ring region 12 and p peripheral region 8 are formedin a surface layer of the n drift layer 2, through implantation of boronions and subsequent thermal diffusion, as shown in FIG. 9(a). Theacceleration voltage for ion implantation is 45 keV, and the dose amountis 5×10¹³ cm⁻². After the ion implantation, annealing is conducted at1150° C. for 200 min. The surface impurity concentration of the p ringregion 12 and p peripheral region 8 is about 2×10¹⁷ cm⁻³, and thediffusion depth is 3 μm.

[0114] Subsequently, the oxide film 6 located inside the p ring region12 is removed by photolithography, and the n⁻ high-resistance region 17is formed by implantation of boron ions and thermal diffusion, as shownin FIG. 9(b). The acceleration voltage for ion implantation is 45 keV,and the dose amount is 1×10¹² cm⁻². After the ion implantation,annealing is conducted at 450° C. for 30 min. Since the dose amount ofboron ions is small, the n⁻ high-resistance region 17 having highresistance, rather than a p region having a reversed conductivity type,is formed in the surface layer of the n drift layer 2. The activationrate of the implanted impurities is as low as about 0.1 because of thelow annealing temperature, and the resistivity of the resulting surfacelayer is about 2 Ω·cm. Then, 0.5 μm-thickness molybdenum which providesa barrier metal and a 5 μm-thickness aluminum alloy are successivelydeposited by sputtering on the surface of the structure shown in FIG.9(b), and patterned by photolithography to thus form the Schottkyelectrode 15 and peripheral electrode 11, respectively, as shown in FIG.9(c).

[0115] Subsequently, a nitride film is deposited by a plasma CVD method,and patterned by photolithography, to provide a protective film 7, andthree layers of titanum, nickel and gold are vapor-deposited on the rearsurface of the n⁺ cathode layer 1, to thus form the cathode electrode 4,as shown in FIG. 9(d).

[0116] The graph of FIG. 8 shows a profile of the resistivity measuredalong a cross section of the Schottky diode 205 of FIG. 7. Thehorizontal axis indicates the depth as measured from the surface of thesemiconductor substrate, and the vertical axis indicates the resistivityas plotted on a logarithmic scale. The n⁻ high-resistance region 17having a depth of 0.3 μm lies at the surface of the semiconductorsubstrate, and the n drift layer 2 having a thickness of about 7 μm andthe n⁺ cathode layer 1 having a low resistivity are present in thisorder under the n⁻ high-resistance region 17. A portion of the surfaceof the high-resistance region 17 having the lowest resistance has aresistivity of about 2 Ω·cm, which is larger than 0.55 Ω·m that is theresistivity of the n drift layer 2.

[0117] In the Schottky diode 206 of the fifth embodiment, the portion ofthe semiconductor substrate which contacts with the Schottky electrode15 has a higher resistivity than that of the known Schottky diode, andtherefore the leakage current I_(R) is reduced to about ½ of that of theknown diode. In addition, since the resistivity of the n drift layer 2is relatively low, the forward voltage VF is lowered by about 0.04 Vupon application of the rated current, as compared with that of theknown Schottky diode. The reverse recovery time t_(rr) is substantiallythe same as that of the known diode. In the case of a Schottky diode of30V-class breakdown voltage, the resistivity and thickness of the ndrift layer 2 are controlled to 0.330Ω·cm and 6 μm, respectively, andthe maximum resistivity of the n⁻ high-resistance region is controlledto 1 Ω·cm, so as to provide similar effects.

[0118] Sixth Embodiment

[0119]FIG. 10 is a cross-sectional view of a Schottky diode 206 having aSchottky barrier according to the sixth embodiment of the presentinvention. Although the basic structure of the Schottky diode 206 of thesixth embodiment is the same as that of the Schottky diode 205 of thefifth embodiment, these diodes 205, 206 are fabricated in differentmethods. As a result, an n⁻ high-resistance region 17 a is formedoutside the p ring region 12 as well as inside this region.

[0120]FIG. 12(a) through FIG. 12(d) are cross-sectional views showingmain process steps of a method for manufacturing the Schottky diode 206of the sixth embodiment. An n drift layer 2 having a resistivity of 0.55Ω·cm and a thickness of 7 μm is laminated by epitaxial growth on an n⁺cathode layer 1 as a substrate, which has a resistivity of 0.0040 Ω·cmand a thickness of 350 μm, and the n⁻ high-resistance region 17 a havinga resistivity of 0.9 Ω·cm and a thickness of 2 μm is then laminated byepitaxial growth on the n drift layer 2, as shown in FIG. 12(a).

[0121] A 1 μm-thickness oxide film 6 is formed by CVD method on theepitaxial wafer thus obtained, and patterned by photolithography. Then,a thin oxide film is formed, and a p ring region 12 and a p peripheralregion 8 are formed in a surface layer of the n drift layer 2, throughimplantation of boron ions and thermal diffusion, as shown in FIG.12(a). The conditions for ion implantation are substantially the same asthose of the fifth embodiment.

[0122] Subsequently, molybdenum and aluminum alloy are deposited bysputtering on the surface of the structure shown in FIG. 12(b), andpatterned by photolithography to thus form a Schottky electrode 15 and aperipheral electrode 11, respectively, such that the Schottky electrode15 is in contact with the n⁻ high-resistance region 17 a, as shown inFIG. 12(c).

[0123] Then, a nitride film is deposited by a plasma CVD method, andpatterned by photolithography, to provide a protective film 7, and threelayers of titanum, nickel and gold are vapor-deposited on the rearsurface of the n⁺ cathode layer 1, to thus form a cathode electrode 4,as shown in FIG. 12(d).

[0124] The graph of FIG. 11 shows a profile of the resistivity measuredalong a cross section of the Schottky diode 206 of FIG. 10. Thehorizontal axis indicates the depth as measured from the surface of thesemiconductor substrate, and the vertical axis indicates the resistivityas plotted on a logarithmic scale. The n⁻ high-resistance region 17 ahaving a depth of 1.5 μm lies at the surface of the semiconductorsubstrate, and the n drift layer 2 having a thickness of about 6 μm andthe n⁺ cathode layer 1 having a low resistivity are present in thisorder under the n- high-resistance region 17 a. Since the n⁻high-resistance region 17 a is laminated by epitaxial growth on the ndrift layer 2, this region 17 a provides an almost constant resistivity,which is larger than 0.55 Ω·m that is the resistivity of the n driftlayer 2.

[0125] The Schottky diode 206 fabricated in the method as describedabove has a reduced leakage current I_(R), and a low forward voltageV_(F).

[0126] Seventh Embodiment

[0127]FIG. 13 is a cross-sectional view of a Schottky diode 202constructed according to the seventh embodiment of the presentinvention. This diode 202 is different from the Schottky diode 205 ofFIG. 7 in that n⁻ high-resistance regions 16 are formed in selectedareas of a surface layer of the n drift layer 2. In the manufacturingmethod of FIG. 9, a step of forming and patterning a photoresist or anoxide film is added prior to the step of implanting boron ions forforming the n⁻ high-resistance region 17, so that the n⁻ high-resistanceregions 17 are formed in selected regions of the n drift layer 2. TheSchottky electrode 15 extends in parallel with the n⁻ high-resistanceregions 17 and exposed portions of the n drift layer 2 having a smallerresistivity than these regions 17, to thus form parallel Schottkydiodes.

[0128] In the present embodiment, the balance of the forward voltageV_(F) and the leakage current I_(R) may be suitably adjusted orcontrolled by changing the area over which the n⁻ high-resistance region17 is formed.

[0129] Eighth Embodiment

[0130]FIG. 14 is a cross-sectional view of a Schottky diode 208 as theeighth embodiment of the present invention, which is a modified exampleof the Schottky diode 205 of FIG. 7. In this example, trenches 16 areformed in a surface layer of the n drift layer 2, and an n⁻high-resistance region 17 is formed on the surface of the n drift layer2 and the inner faces of the trenches 16, through implantation of boronions and thermal diffusion. Schottky diode 15 made of molybdenum, forexample, is formed in contact with the n⁻ high-resistance region 17.With the trenches 16 thus provided, the contact area of the Schottkyelectrode 15 is increased, with a result of an increase in the currentcapacitance.

[0131] Ninth Embodiment

[0132]FIG. 15 is a cross-sectional view of a composite diode 209 of theninth embodiment of the present invention having pn junction andSchottky junction. A semiconductor substrate consists of ahigh-concentration n⁺ cathode layer 1 and a low-concentration n driftlayer 2, and p⁻ anode regions 3 a are formed in selected areas of asurface layer of the n drift layer 2 of the substrate. On thesemiconductor substrate is formed a Schottky electrode 15 made of ametal, such as molybdenum, which forms high Schottky junctions orbarriers when contacting with exposed portions of the n drift layer 2.Molybdenum is also in ohmic contact with the surfaces of the p⁻ anoderegions 3 a. Schottky electrodes may be formed on the exposed portionsof the n drift layer 2, and a metal that is in ohmic contact with thesurfaces of the p⁻ anode regions 4 a may be superposed on the Schottkyelectrodes. A cathode electrode 3 is formed on the rear surface of then⁺ cathode layer 1. The Schottky diode 209 further includes aring-shaped p ring region 12, oxide film 6 that covers the surface ofthe pn junction, and a protective film 7 in the form of a nitride film.A p-type peripheral region 8 is formed in a peripheral portion of thecomposite diode 209, and a peripheral electrode 11 is formed on thesurface of the p peripheral region 8, to extend over a part of the oxidefilm 6. The p⁻ anode region 3 a has substantially the same dimension(depth) and concentration distribution as the p⁻ anode region 3 a of thepn diode 201 of the first embodiment.

[0133] The composite diode 209 having the pn junction and Schottkyjunctions can make use of a low forward-voltage characteristic of theSchottky diode in a low current region and that of the pn junction diodein a high current region. During reverse recovery, this composite diode209 takes advantage of a fast switching characteristic (high switchingspeed) of the Schottky diode. In the present embodiment, the totalamount of impurities contained in the p⁻ anode regions 3 a is reduced byabout three orders of magnitude, and therefore the amount of minoritycarriers injected into the n drift layer 2 is considerably reduced,resulting in a further shortened reverse recovery time t_(rr).

[0134] In order to control the reverse recovery time t_(rr) to a givenvalue, the composite diode 209 requires a considerably reduced amount oflifetime killers, and thus the leakage current I_(R) can besignificantly reduced. In addition, the reverse recovery waveformexhibits soft recovery.

[0135] If the spacing between adjacent ones of the p⁻ anode regions 3 ais reduced, depletion layers formed in the regions 3 a may connect witheach other upon application of reverse bias, resulting in furtherreduction in the leakage current.

[0136] Tenth Embodiment

[0137]FIG. 16 is a cross-sectional view of a composite diode 210 of thetenth embodiment of the present invention, which is a slightly modifiedexample of the composite diode 209 of FIG. 15. In this example, a panode region 3 having a large total amount of impurities is formed in asurface layer of an n drift layer 2, and trenches 16 having a largerdepth than the p anode region 3 are formed in the n drift layer 2,through the p anode region 3. Further, n⁻ high-resistance regions 17having a small diffusion depth are formed on the inner faces of portionsof the trenches 16 that have a larger depth than the p anode region 3,as in the fifth embodiment. A Schottky electrode 15 made of a suitablemetal is embedded in the trenches 16 such that the metal contacts withthe surfaces of the n⁻ high-resistance regions 17 on the inner faces ofthe trenches 16, so as to form Schottky junctions. The Schottkyelectrode 15 is also in ohmic contact with the surface of the p anoderegion 3. Since the resistivity of a contact portion of the Schottkyelectrode 15 of this diode 210 is higher than that of the known Schottkydiode, the leakage current I_(R) can be reduced, and the resistivity ofthe n drift layer 2 is kept at a low level, resulting in a reducedforward voltage V_(F). Moreover, the provision of the trenches 16 leadsto an increase in the total area of Schottky junctions formed by theSchottky electrode 15 and the n⁻ high-resistance regions 17 having asmall diffusion depth, and an increase in the current capacitance. Atthe same time, the provision of the p anode region 3 having a largetotal amount of impurities leads to reduction in the forward voltage ina high current region.

[0138] Eleventh Embodiment

[0139]FIG. 17 is a cross-sectional view of a pn junction diode 211constructed according to the eleventh embodiment of the presentinvention. In this diode 211, an n⁺ embedded region 18 is formed in apart of the interface between a high-concentration n⁺ cathode layer 1and a low-concentration n drift layer 2, and a p anode region 3 areformed in a selected area of a surface layer of the n drift layer 2above the n⁺ embedded region 18. What is important in this embodiment isthat the width L of the p anode region 3 is larger than the width 1 ofthe n⁺ embedded region 18. Anode electrode 5 and cathode electrode 4 areformed in contact with the surface of the p anode region 3 and the rearsurface of an n⁺ cathode layer 1, respectively. The diode 211 furtherincludes an oxide film 6 that covers the surface of the pn junction, anda protective film 7 in the form of a nitride film. A p peripheral region8 is formed in a peripheral portion of the pn diode 211, and aperipheral electrode 11 is formed on the surface of the p peripheralregion 8. The peripheral electrode 11 also extends over a part of theoxide film 6.

[0140]FIG. 19(a) through FIG. 19(d) are cross-sectional views showingmain process steps of a method for manufacturing the pn junction diode211 of the eleventh embodiment. A substrate that has a resistivity of0.004 Ω·cm and a thickness of 350 μm and provides the n⁺ cathode layer 1is thermally oxidized, so that an oxide film 6 a is grown on thesubstrate. Then, the oxide film 6 a is patterned by photolithography,and phosphorous ions for forming the n⁺ embedded region 18 are implantedin the substrate, as shown in FIG. 19(a). Reference numeral 18 a denotesan ion implantation layer. The acceleration voltage during ionimplantation is 45 keV, and the dose amount is 5×10¹⁴ cm⁻².

[0141] Subsequently, the oxide film 6 a is removed, and the n driftlayer 2 is laminated by epitaxial growth on the n⁺ cathode layer 1 andthe ion implantation layer 18 a, and heat-treated, thereby to form then⁺ embedded region 18, as shown in FIG. 19(b). The n drift layer 2 has aresistivity of 40 Ω·m and a thickness of 60 μm.

[0142] The oxide film 6 is formed by thermally oxidizing the epitaxialwafer thus formed, and patterned by photolithography to provide a mask,and the p anode region 3 and p peripheral region 8 are formed in asurface layer of the n drift layer 2, using the mask, throughimplantation of boron ions and thermal diffusion, as shown in FIG.19(c). The acceleration voltage for ion implantation is 45 keV, and thedose amount is 5×10¹⁴ cm⁻². After the ion implantation, annealing isconducted at 1150° C. for 200 min. The surface impurity concentration ofthe p anode region 3 and p peripheral region 8 is about 1×10¹⁹ cm⁻³, andthe diffusion depth is 3 μm.

[0143] A 5 μm-thickness aluminum alloy is deposited by sputtering on thesurface of the structure shown in FIG. 19(c), and patterned byphotolithography to thus form the anode electrode 5 and peripheralelectrode 11, as shown in FIG. 19(d).

[0144] Subsequently, a nitride film is deposited by a plasma CVD method,and patterned by photolithography, to provide the protective film 7, andthree layers of titanum, nickel and gold are vapor-deposited on the rearsurface of the n⁺ cathode layer 1, to thus form the cathode electrode 4,as shown in FIG. 19(e).

[0145]FIG. 20(a) shows an example of a photomask used for forming the n⁺embedded region. The n⁺ embedded region having a rectangular shape isformed in the hatched portion in FIG. 20(a). Another example ofphotomask is shown in FIG. 20(b) which is used for forming alattice-shaped n⁺ embedded region, and other examples of photomasks areshown in FIG. 20(c) and FIG. 20(d) which are used for forming spaced n⁺embedded regions.

[0146] The graph of FIG. 18 shows a profile of the resistivity measuredalong a cross section of the pn junction diode 211 of FIG. 17. Thehorizontal axis indicates the depth as measured from the surface of thesemiconductor substrate, and the vertical axis indicates the resistivityas plotted on a logarithmic scale. The p anode region 3 having a depthof 0.3 μm lies at the surface of the semiconductor substrate, and the ndrift layer 2 having a thickness of about 53 μm, and the n⁺ embeddedregion and n⁺ cathode layer 1 each having a low resistivity are presentin this order under the p anode region 3.

[0147] In the pn junction diode 211 of the eleventh embodiment, thewidth L of the p anode region 3 is larger than the width 1 of the n⁺embedded region 18. When a reverse bias is applied to this diode,therefore, a depletion layer that spreads from the pn junction betweenthe p anode region 3 and the n drift layer 2 initially hits the n⁺embedded region 18. Accordingly, the avalanche breakdown does not occurat a portion of the p anode region 2 having a certain radius ofcurvature, or its corner portion, but occurs uniformly in asmall-thickness region of the n drift layer 2. Thus, the breakdown takesplace occurs at the same time over a broad area or region, and thereforethe diode has an increased avalanche current.

[0148] For example, a 3 mm-square diode having a rated voltage of 600V,which is constructed according to the eleventh embodiment, has anavalanche current of 100A, which is at least 10 times greater than thatof the known diode.

[0149] Twelfth Embodiment

[0150]FIG. 21(a) through FIG. 21(d) are cross-sectional views showingmain process steps of a method for manufacturing a n junction diode 212according to the twelfth embodiment of the present invention. Initially,an n drift layer 2 a having a resistivity of 40 Ω·m and a thickness of10 μm is formed by epitaxial growth on a substrate which provides an n⁺cathode layer 1. The substrate is doped with phosphorous ions, and has aresistivity of 0.004 Ω·cm and a thickness of 350 μm. The epitaxial waferthus formed is thermally oxidized, so that an oxide film 6 a is grown onthe wafer. Then, the oxide film 6 a is patterned by photolithography,and antimony ions for forming an n⁺ embedded region are implanted, andheat-treated, to thus form an n⁺ embedded region 18, as shown in FIG.21(a). The acceleration voltage for ion implantation is 45 keV, and thedose amount is 5×10¹⁴ cm⁻². The heat treatment is conducted at 1150° C.for 300 min.

[0151] Subsequently, the oxide film 6 a is removed, and an n drift layer2 b having a resistivity of 40 Ω·cm is laminated by epitaxial growthwith an addition thickness of 50 μm, as shown in FIG. 21(b). The totalthickness of the n drift layer 20 is 60 μm when measured at its thinportion, and 70 μm when measured at its thick portion.

[0152] An oxide film 6 is formed by thermally oxidizing the epitaxialwafer thus formed, and patterned by photolithography, to provide a mask,and a p anode region 3 and a p peripheral region 8 are formed in asurface layer of the n drift layer 2 b, using the mask, throughimplantation of boron ions and thermal diffusion, as shown in FIG.21(c). The acceleration voltage for ion implantation is 45 keV, and thedose amount is 5×10¹³ cm⁻². After the ion implantation, annealing isconducted at 1150° C. for 200 min. The surface impurity concentration ofthe p anode region 3 and p peripheral region 8 is about 1×10¹⁷ cm⁻³, andthe diffusion depth of these regions is 3 μm.

[0153] Thereafter, anode electrode 5, peripheral electrode 11, cathodeelectrode 4 and protective film 7 are formed in the same manner as inthe eleventh embodiment, to provide a structure as shown in FIG. 21(d).

[0154] According to the manufacturing method as described above, the n⁺embedded region 18 a having a large thickness can be easily formed, andits thickness can be easily controlled. Another advantage of this methodis that the n drift layer 2 is heat-treated to a reduced degree.

[0155] In this embodiment, too, the width of the p anode region 3 islarger than the width of the n⁻ embedded region 18 a. Accordingly, theavalanche breakdown of the pn junction between the p anode region 3 andthe n drift layer 2 occurs at a small-thickness region of the n driftregion 2, and thus the avalanche current of the diode can be increased.

[0156] Thirteenth Embodiment

[0157]FIG. 22 is a cross-sectional view of a pn junction diode 213constructed according to the thirteenth embodiment of the presentinvention. As in the eleventh embodiment, an n⁺ embedded region 18 isformed in a part of the interface between a high-concentration n⁺cathode layer 1 and a low-concentration n drift layer 2. A p ring region12 is formed in a surface layer of the n drift layer 2 above the n⁺embedded region 18, and a p anode region 3 having a smaller diffusiondepth than the p ring region 12 is formed inside the p ring region 12.For example, when the diffusion depth of the p ring region 12 is 5 μm,the diffusion length of the p anode region is controlled to 3 μm. Inthis embodiment, it is considered important that the spacing L betweenopposite portions of the p ring region 12 is set to be larger than thewidth 1 of the n⁺ embedded region 18, and that the thickness of the ndrift layer 2 between the p anode region 3 and the n⁺ embedded region 18is smaller than that of the n drift layer 2 located below the p ringregion 12.

[0158] With this arrangement, when a reverse-bias voltage is applied, adepletion layer that extends from the p anode region 3 reaches the n⁺embedded region 18 before a depletion layer that extends from the p ringregion 12 reaches the n⁺ cathode region 1. As a result, breakdown occursuniformly in a small-thickness region of the n drift layer 2, and thusthe avalanche current of the diode 213 can be increased.

[0159] Fourteenth Embodiment

[0160]FIG. 23 is a cross-sectional view of a pn junction diode 214constructed according to the fourteenth embodiment of the presentinvention. As in the twelfth embodiment, an n⁺ embedded region 18 isformed in a part of the interface between a high-concentration n⁺cathode layer 1 and a low-concentration n drift layer 1, and a p ringregion 12 is formed in a surface layer of the n drift layer 2. In thepresent embodiment, a p⁻ anode region 3 a similar to that of the firstembodiment, which has a small total amount of impurities and a smalldiffusion depth, is formed inside the p ring region 12. For example, thedose amount of boron ions when forming the p⁻ anode region 3 a is 1×10¹²cm⁻², and the depth of the pn junction between the p⁻ anode region 3 aand the n drift layer 2 is 0.3 μm. It is considered important that thespacing between opposite portions of the p ring region 12 is larger thanthe width of the n⁺ embedded region 18, and that the thickness of the ndrift layer 2 between the p⁻ anode region 3 a and the n⁺ embedded region18 is smaller than that of the n drift layer 2 located below the p ringregion 18.

[0161] In this embodiment, too, breakdown of the pn junction between thep anode region 3 a and the n drift layer 2 occurs uniformly in asmall-thickness region of the n drift layer 2, which leads to anincreased avalanche current of the diode 214, as in the thirteenthembodiment. Further, as in the first embodiment, the total amount ofimpurities contained in the p⁻ anode region 3 a is reduced by aboutthree orders of magnitude as compared with that of the known pn junctiondiode, whereby the amount of minority carriers injected into the n driftlayer 2 is considerably reduced, and the reverse recovery time t_(rr) isaccordingly shortened.

[0162] In order to control the reverse recovery time t_(rr) to a givenvalue, lifetime killers may be significantly reduced as compared withthose of the known diode, and therefore the leakage current I_(R) can begreatly reduced. In addition, the reverse recovery waveform exhibitssoft recovery owing to increased lifetime in the n drift layer 2.

[0163] Fifteenth Embodiment

[0164]FIG. 24 is a cross-sectional view of a pn junction diode 215constructed according to the fifteenth embodiment of the presentinvention. As in the twelfth embodiment, an n⁺ embedded region 18 isformed in a part of the interface between a high-concentration n⁺cathode layer 1 and a low-concentration n drift layer 1, and a p ringregion 12 is formed in a surface layer of the n drift layer 2. In thepresent embodiment, a p⁻ anode region 3 a similar to that of thefourteenth embodiment, which has a low surface impurity concentrationand a small diffusion depth, is formed inside the p ring region 12.Further, p high-concentration regions 13 having a high surfaceconcentration and a large diffusion depth are formed between spacedportions of the p⁻ anode region 3 a. When rated current is applied tothis diode 215, the current flows through the p⁻ anode region 3 a,assuring an excellent reverse recovery characteristic. When a reversebias is applied, a depletion layer extends from the p high-concentrationregion 13, and thus the resulting diode has a high breakdown voltage.

[0165] In the pn junction diode 12, the spacing between oppositeportions of the p ring region 12 is larger than the width of the n⁺embedded region 18, and the thickness of the n drift layer 2 between thep high-concentration region 13 and the n⁺ embedded region 18 is smallerthan the thickness of the n drift layer 2 located below the p ringregion 12. When a reverse-bias voltage is applied, therefore, thebreakdown occurs at the same time at a region inside the p ring region12. Thus, the diode 215 has an increased avalanche current. The phigh-concentration regions 13 may be formed in the shape of stripes,lattice, or dots.

[0166] Sixteenth Embodiment

[0167]FIG. 25 is a cross-sectional view of a pn junction diode 216constructed according to the sixteenth embodiment of the presentinvention. An n⁺ embedded region 18 is formed in a part of the interfacebetween a high-concentration n⁺ cathode layer 1 and a low-concentrationn drift layer 1, and trenches 16 are formed in a surface layer of the ndrift layer 2. A p⁻ anode region 3 a similar to that of the firstembodiment, which has a small total amount of impurities and a smalldiffusion depth, is formed along the surface of the n drift layer 2 andinner faces of the trenches 16, and an anode electrode 5 is formed incontact with the p⁻ anode region 3 a. Namely, the diode 216 of thepresent embodiment is obtained by adding the n⁺ embedded region 18 tothe pn junction diode of the fifth embodiment. The spacing between theoutermost trenches 16 is made larger than the width of the n⁺ embeddedregion 18.

[0168] In a conventional diode provided with trenches, concentration ofthe electric field takes place at a portion where the outermost trenchesare located, resulting in a reduced breakdown voltage. This is becauseeach trench located inside the outermost trenches faces two trenches onthe opposite sides thereof, whereas each of the outermost trenches facesonly one trench located on one of the opposite sides thereof.

[0169] In the present embodiment in which the concentration of theelectric field at the outermost trenches is avoided, when a reverse-biasvoltage is applied, the breakdown of the pn junction between the p⁻anode region 3 a and the n drift layer 2 occurs at the same time at thetrenches 18 located inside the outermost trenches, and thus theavalanche current is increased. Further, as in the first embodiment, thetotal amount of impurities contained in the p⁻ anode region 3 a isreduced by about three orders of magnitude, as compared with that of theknown pn junction diode, and therefore the amount of minority carriersinjected into the n drift layer 2 is remarkably reduced, with a resultof reduction in the reverse recovery time t_(rr). In addition, theprovision of the trenches 16 lead to an increase in the contact area ofthe pn junction, and an increase in the current capacitance.

[0170] The trenches 16 may be formed in the shape of stripes or lattice.

[0171] Seventeenth Embodiment

[0172]FIG. 26 is a cross-sectional view of a pn junction diode 217constructed according to the seventeenth embodiment of the presentinvention. An n⁺ embedded region 18 is formed in a part of the interfacebetween a high-concentration n⁺ cathode layer 1 and a low-concentrationn drift layer 2, and a p anode region 3 having a large total amount ofimpurities is formed in a surface layer of the n drift layer 2. Further,trenches 16 having a larger depth than the p anode region 3 are formedin the n drift layer 2, through the p anode region 3, and p⁻ anoderegions 3 a similar to that of the first embodiment are formed on theinner faces of portions of the trenches 16 which are deeper than the panode region 3. The total amount of impurities in the p⁻ anode region 3a is smaller by some orders of magnitude than that of the known diode,and this region 3 a has a relatively small diffusion depth. Namely, thepn junction diode 217 of this embodiment is obtained by adding the n⁺embedded region 18 to the pn junction diode of the fourth embodimentshown in FIG. 6. The width or spacing between the outermost trenches islarger than the width of the n⁺ embedded region 18.

[0173] In the present embodiment, too, when a reverse-bias voltage isapplied, the breakdown occurs at the same time at a portion of the ndrift layer 2 below the inner trenches 16 located inside the p ringregion 12, and thus the avalanche current is increased. With the p anoderegion 3 a and p anode region 3 thus provided, the forward voltage canbe reduced in a low current region and a high current region, and theswitching time can be shortened. Also, the provision of the trenches 15leads to an increase in the contact area of the p⁻ anode region 3 ahaving a small diffusion depth, and an increase in the currentcapacitance.

[0174] Eighteenth Embodiment

[0175]FIG. 27 is a cross-sectional view of a Schottky diode 218constructed according to the eighteenth embodiment. An n⁺ embeddedregion 18 is formed in a part of the interface between ahigh-concentration n⁺ cathode layer 1 and a low-concentration n driftlayer 2, and a p ring region 12 is formed in a surface layer of the ndrift layer 2. A Schottky electrode 15 is provided which contacts withthe surface of the n drift layer 2 inside the p ring region 12, to forma Schottky junction.

[0176] The n drift layer 2 has a resistivity of 0.9 Ω·m and a thicknessof 7 μm. The Schottky electrode 15 is formed of molybdenum, for example.Namely, the Schottky diode 218 of the present embodiment is obtained byadding the n⁺ embedded region 18 to the known Schottky diode of FIG. 41.The width of the n drift layer 2 between opposite portions of the p ringregion 12 is larger than the width of the n⁺ embedded region 18, and thethickness of the n drift layer 2 located on the n⁺ embedded region 18 issmaller than that of the n drift layer 2 under the p ring region 12.

[0177] In this arrangement, when a reverse bias is applied, a depletionlayer initially reaches the n⁺ embedded region 18, and the breakdownoccurs uniformly in a small-thickness wide region of the n drift layer2. Thus, the avalanche current of the diode can be increased.

[0178] For example, a 3 mm-square diode having a rated voltage of 60V,which is constructed according to the eighteenth embodiment, has anavalanche current of 60A, which is at least 10 times greater than thatof the known diode.

[0179] Nineteenth Embodiment

[0180]FIG. 28 is a cross-sectional view of a Schottky diode 219constructed according to the nineteenth embodiment of the presentinvention. A n⁺ embedded region 18 is formed in a part of the interfacebetween a high-concentration n⁺ cathode layer 1 and a low-concentrationn drift layer 2, and a p ring region 12 is formed in a surface layer ofthe n drift layer 2. An n⁻ high-resistance region 17 having a very smallthickness is also formed in the surface layer of the n drift layer 2inside the p ring region 12, and a Schottky electrode 15 is formed incontact with the n⁻ high-resistance region 17 and the p ring region 12.

[0181] The n drift layer 2 has a resistivity of 0.550Ω·cm, and athickness of 7 μm. The n⁻ high-resistance region 17 is formed byimplantation of boron ions, and has the highest resistivity of about 2Ω·cm, and a thickness of 0.5 μm. The Schottky electrode 15 is formed ofmolybdenum. Namely, the Schottky diode 219 of the present embodiment isobtained by adding the n⁺ embedded region 18 to the Schottky diode offifth embodiment shown in FIG. 7. The spacing between opposite portionsof the p ring region 12 is larger than the width of the n⁺ embeddedregion 18. The thickness of the n drift layer 2 located above the n⁺embedded region 18 is smaller than that of the n drift layer 2 under thep ring region 12.

[0182] Accordingly, as in the Schottky diode of the fifth embodiment, aportion of this diode 219 (corresponding to the n⁻ high-resistanceportion 17) which contacts with the Schottky electrode 15 has a higherresistivity than that of the known Schottky diode, and the leakagecurrent I_(R) is reduced to about one half, assuring an equivalentforward voltage. Further, when a reverse bias is applied, a deletionlayer initially reaches the n⁺ embedded region 18, and therefore thebreakdown occurs uniformly in a small-thickness region of the n driftlayer 2. Thus, the avalanche current of the present diode is increased.

[0183] Twentieth Embodiment

[0184]FIG. 29 is a cross-sectional view of a Schottky diode 220constructed according to the twentieth embodiment of the presentinvention. In this embodiment, an n⁺ embedded region 18 formed in a partof the interface between a high-concentration n⁺ cathode layer 1 and alow-concentration n drift layer 2, and a p ring region 12 is formed in asurface layer of the n drift layer 2. An n⁻ high-resistance region 17having a very small thickness is also formed in the surface layer of then drift layer 2 inside the p ring region 12, and a Schottky electrode 15is formed in contact with the n⁻ high-resistance region 17 and the pring region 12. The Schottky diode 220 of the present embodiment isdifferent from the Schottky diode 219 of the nineteenth embodiment inthat the n⁻ high resistance region 17 having a low concentration isformed by epitaxial growth, to extend over the surface of the n driftlayer 2 outside the p ring region 12. The n drift layer 2 has aresistivity of 0.550Ω·cm, and a thickness of 6 μm, and the n⁻high-resistance region 17 is has a resistivity of 0.9 Ω·cm, and athickness of 1 μm. The Schottky electrode 15 is formed of molybdenum.

[0185] Accordingly, like the Schottky diode 219 of the nineteenthembodiment, the Schottky diode 220 has a reduced leakage current I_(R)and a large avalanche current.

[0186] Twenty-First Embodiment

[0187]FIG. 30 is a cross sectional view of a Schottky diode 221constructed according to the twenty-first embodiment of the presentinvention, which is a modified example of the Schottky diode 219 of thenineteenth embodiment. In this example, n⁻ high-resistance regions 17having a very small thickness are formed in selected areas of a surfacelayer of the n drift layer 2 inside the p ring region 12. In a method offabricating this diode 221, a step of patterning a photoresist or oxidefilm to provide a mask is added prior to the step of implanting boronions to form the n⁻ high-resistance region 7, so that the ions areimplanted in selected areas defined by the mask. The spacing betweenopposite portions of the p ring region 12 is larger than the width ofthe n⁺ embedded region 18. The thickness of the n drift layer 2 locatedon the n⁺ embedded region 18 is smaller than the thickness of the ndrift layer under the p ring region 12.

[0188] Namely, the Schottky diode 221 of the present embodiment isobtained by adding the n⁺ embedded region 18 to the Schottky diode 207of the seventh embodiment shown in FIG. 13. Accordingly, in addition tothe effects of the Schottky diode of the seventh embodiment as describedabove, the breakdown occurs uniformly in a small-thickness region of then drift layer 2, and therefore the avalanche current is increased.

[0189] Twenty-Second Embodiment

[0190]FIG. 31 is a cross-sectional view of a Schottky diode 222constructed according to the twenty-second embodiment of the presentinvention. The Schottky diode 222 of this embodiment is obtained byadding an n⁺ embedded region 18 to the known Schottky diode withtrenches as shown in FIG. 42.

[0191] With the trenches 16 thus provided, the contact area of theSchottky electrode 15 is increased, with a result of an increase in thecurrent resistance. In addition, the spacing between the outermosttrenches 16 is larger than the width of the n⁺ embedded region 18, thusavoiding concentration of the electric field at the outermost trenches,and the breakdown occurs in a small-thickness region of the n driftlayer 2. Thus, the Schottky diode 222 of the present embodiment has anincreased avalanche current.

[0192] Twenty-Third Embodiment

[0193]FIG. 32 is a cross-sectional view of a Schottky diode 223constructed according to the twenty-third embodiment of the presentinvention. The diode 223 of the present embodiment is obtained by addingan n⁺ embedded region 18 to the Schottky diode 208 of the eighthembodiment shown in FIG. 14.

[0194] Accordingly, the leakage current I_(R) can be reduced due to thehigh resistivity of the n⁻ high-resistance region 17 that contacts withthe Schottky electrode 15, and the contact area of the Schottkyelectrode 15 is increased by providing the trenches 16, thus assuringincreased current capacitance. In addition, since the spacing betweenthe outermost trenches 16 is larger than the width of the n⁺ embeddedregion 18, the breakdown occurs uniformly in a small-thickness region ofthe n drift layer 2. Thus, the Schottky diode 223 has an increasedavalanche current.

[0195] Twenty-Fourth Embodiment

[0196]FIG. 33 is a cross-sectional view of a composite diode 224constructed according to the twenty-fourth embodiment of the presentinvention. The diode 24 of this embodiment is obtained by adding an n⁺embedded region 18 to the known composite diode of FIG. 44. A p ringregion 12 is formed in a surface layer of an n drift layer 2, and phigh-concentration regions 13 having a high surface concentration and alarge diffusion depth are formed inside the p ring region 12, such thatexposed surfaces of the n drift layer 2 interposed between the phigh-concentration regions 13 are in contact with a Schottky electrode15. Also, the Schottky electrode 15 is in ohmic contact with the phigh-concentration regions 13.

[0197] In this arrangement, the composite diode 224 having the pnjunction and Schottky junction provides a lowered forward voltage as acomposite effect of the pn junction diode and Schottky diode, and a fastswitching characteristic (high switching speed) as an effect of theSchottky diode. Further, the spacing between opposite portions of the pring region 12 is larger than the n⁺ embedded region 18, and thethickness of the n drift layer on the n⁺ embedded region 18 is smallerthan that of the n drift layer 2 below the p ring region 12.Accordingly, the breakdown occurs uniformly in a small-thickness portionof the n drift layer 2, and the avalanche current of the diode isincreased.

[0198] If the width of each of the exposed portions of the n drift layer2 between the p anode regions 3 is reduced, a depletion layer spreadsout from the p anode regions 3 upon application of a reverse bias, whichyields an effect of reducing the leakage current.

[0199] Twenty-Fifth Embodiment

[0200]FIG. 34 is a cross-sectional view of a composite diode 225constructed according to the twenty-fifth embodiment of the presentinvention. The diode 225 of the present embodiment is obtained by addingan n⁺ embedded region 18 to the composite diode 209 of the ninthembodiment shown in FIG. 15. The spacing between opposite portions ofthe p ring region 12 is larger than the width of the n⁺ embedded region18. Also, the thickness of the n drift layer 2 on the n⁺ embedded region18 is smaller than the thickness of the n drift layer 2 under the p ringregion 12.

[0201] Accordingly, the composite diode 225 exhibits a lowforward-voltage characteristic, a high switching speed and a reducedleakage current as provided by the composite diode 209 of the ninthembodiment. In addition, the diode 225 has an increased avalanchecurrent since the breakdown occurs uniformly in a small-thickness regionof the n drift layer 2.

[0202] Twenty-Sixth Embodiment

[0203]FIG. 35 is a cross-sectional view of a composite diode 226constructed according to the twenty-sixth embodiment of the presentinvention. The diode 226 of this embodiment is obtained by adding an n⁺embedded region 18 to the known composite diode 108 provided withtrenches as shown in FIG. 45. The spacing between the outermost trenches16 is larger than the width of the n⁺ embedded region 18.

[0204] Accordingly, the composite diode 226 exhibits a lowforward-voltage characteristic, a high switching speed, and largecurrent capacitance as provided by the known composite diode 108. Inaddition, the avalanche breakdown occurs uniformly in a small-thicknessregion of the n drift layer 2, and thus the avalanche current isincreased.

[0205] Twenty-Seventh Embodiment

[0206]FIG. 36 is a cross-sectional view of a composite diode 227constructed according to the twenty-seventh embodiment of the presentinvention. The composite diode 226 of the twenty-sixth embodiment ofFIG. 35 is modified into the diode 227 by providing n⁻ high-resistanceregions 17 at portions of the n drift layer 2 that contact with theSchottky electrode 16.

[0207] Accordingly, the composite diode 227 provides a low forwardvoltage, and has a fast switching characteristic and a large currentcapacitance. In addition, the avalanche current is increased since thebreakdown occurs uniformly in a small-thickness region of the n driftlayer 2, and the leakage current I_(R) is also reduced.

[0208] According to the present invention as explained above, in the pnjunction diode including the first-conductivity-type drift layer and thesecond-conductivity-type anode region, the lowest resistivity of thesecond-conductivity-type anode region is at least {fraction (1/100)} ofthat of the first-conductivity-type drift layer, and the thickness ofthe anode region is smaller than that of the second-conductivity-typering region. Preferably, the lowest resistivity of the anode region is0.3 to 30 times the resistivity of the drift layer, and the thickness ofthe anode region is controlled to 0.01 to 0.5 μm. With the diode thusconstructed, the amount of minority carriers injected into thefirst-conductivity-type drift layer is significantly reduced, and thecarriers accumulated in this layer is also reduced, so that theresulting diode has a reduced reverse recovery time. Where the reverserecovery time of the diode is set to a certain value, the leakagecurrent arising upon application of a reverse bias can be significantlyreduced as compared with that of the known diode.

[0209] In a method for manufacturing the diode as described above, thesecond-conductivity-type anode region may be formed by implantingsecond-conductivity-type ions in a dose amount of 1×10¹⁰ to 1×10¹² cm⁻²,and conducting heat treatment at a temperature in a range of 300 to 600°C.

[0210] In the Schottky diode wherein the generally ring-likesecond-conductivity-type ring region is formed in the surface layer ofthe first-conductivity-type drift layer, and the Schottky electrode isformed in contact with the surface of the semiconductor substrate insidethe ring region to form a Schottky junction, the first-conductivity-typehigh-resistance region is formed in the surface layer of the firstconductivity-type drift layer located inside thesecond-conductivity-type ring region, such that the high-resistanceregion has a higher resistivity than the first-conductivity-type driftlayer, and has a thickness that is smaller than a diffusion depth of thesecond-conductivity-type ring region. Preferably, the highestresistivity of the high-resistance region is in the range of 1.2 to 12times the resistivity of the first-conductivity-type drift layer, andits thickness is in the range of 0.01 to 0.5 μm. In this arrangement,the leakage current arising upon application of a reverse bias can bereduced, without deteriorating its characteristics during application ofa forward voltage.

[0211] In a method for manufacturing such a Schottky diode, thefirst-conductivity-type high-resistance region is formed by implantingsecond-conductivity-type ions in a dose amount of 1×10¹⁰ to 1×10¹³ cm⁻²,preferably, in a dose amount of 1×10¹⁰ to 1×10¹² cm⁻², and conductingheat treatment. The first-conductivity-type high-resistance region mayalso be formed by epitaxial growth.

[0212] In the diode including the first-conductivity-type cathode layerhaving a high impurity concentration, first-conductivity-type driftlayer placed on the cathode layer and having a lower impurityconcentration than the cathode layer, and the first-conductivity-typeembedded region sandwiched between the cathode layer and the drift layerand having a lower resistivity than the drift layer, thefirst-conductivity-type embedded region is formed only in the inside ofthe vertical projection of the second-conductivity-type anode region orsecond-conductivity-type ring region. The thus constructed diode has asignificantly improved ability to withstand avalanche breakdown, sincethe breakdown occurs uniformly in a broad area that does not include acurved portion of the pn junction having a certain radius of curvature.

[0213] In the diode in which trenches are formed in the surface layer ofthe first-conductivity-type drift layer, the first-conductivity-typeembedded region is formed only inside the area bounded by the verticalprojection of the outermost trenches, so as to avoid concentration ofthe electric field. This diode has an improved ability to withstandavalanche breakdown.

[0214] The above features of the present invention may be combined toprovide a diode which yields the respective effects of the features atthe same time. While individual diodes corresponding to the respectiveembodiments are shown in the figures, the present invention may beapplied to diodes that are integrated in semiconductor integratedcircuits.

What is claimed is:
 1. A diode comprising: a first-conductivity-typefirst region having a first impurity concentration; afirst-conductivity-type second region placed on said first region andhaving a second impurity concentration that is lower than said firstimpurity concentration; a generally ring-like second-conductivity-typethird region formed in a surface layer of said second region; asecond-conductivity-type fourth region formed in a surface layer of saidsecond region located inside said third region; a first main electrodeformed in contact with said first region; and a second main electrodeformed in contact with said fourth region, wherein a portion of saidsecond-conductivity-type fourth region having the lowest resistance hasa resistivity that is at least {fraction (1/100)} of that of saidfirst-conductivity-type second region, and said second-conductivity-typefourth region has a thickness that is smaller than a diffusion depth ofsaid second-conductivity-type third region.
 2. A diode according toclaim 1, wherein the resistivity of the portion of said fourth regionhaving the lowest resistance is in a range of 0.3 to 30 times that ofsaid second region.
 3. A diode according to claim 1, wherein said fourthregion has a diffusion depth in a range of 0.01 to 0.5 μm.
 4. A methodfor manufacturing a diode, comprising the steps of: preparing afirst-conductivity-type first region having a first impurityconcentration; placing a first-conductivity-type second region on saidfirst region, said second region having a second impurity concentrationthat is lower than said first impurity concentration; forming agenerally ring-like second-conductivity-type third region in a surfacelayer of said second region; forming a second-conductivity-type fourthregion in a surface layer of said second region located inside saidthird region, such that a portion of the fourth region having the lowestresistance has a resistivity that is at least {fraction (1/100)} of thatof said first-conductivity-type second region, and such that the fourthregion has a thickness which is smaller than a diffusion depth of saidsecond-conductivity-type third region; forming a first main electrode incontact with said first region; forming a second main electrode incontact with said fourth region, wherein said fourth region is formed byimplanting second-conductivity-type ions in a dose amount of 1×10¹⁰ to1×10¹² cm⁻², and conducting heat treatment.
 5. A method according toclaim 4, wherein the heat treatment for forming said fourth region isconducted at a temperature in a range of 300-600° C.
 6. A diodecomprising: a first-conductivity-type first region having a firstimpurity concentration; a first-conductivity-type second region placedon said first region and having a second impurity concentration that islower than said first impurity concentration, said first region and saidsecond region constituting a semiconductor substrate; a generallyring-like second-conductivity-type third region formed in a surfacelayer of said second region; a first main electrode formed in contactwith said first region; a third main electrode which contacts with asurface of said semiconductor substrate located inside said thirdregion, so as to form a Schottky junction; and a first-conductivity-typefifth region formed in a surface layer of said second region locatedinside said second-conductivity-type third region, said fifth regionhaving a resistivity that is higher than that of said second region, andhaving a thickness that is smaller than a diffusion depth of said thirdregion.
 7. A diode according to claim 6, wherein the thickness of saidfifth region is in a range of 0.01 to 3.0 μm.
 8. A diode according toclaim 7, wherein the thickness of said fifth region is in a range of0.01 to 0.5 μm.
 9. A diode according to claim 6, wherein a portion ofsaid fifth region having the highest resistance has a resistivity thatis 1.2 to 12 times that of said second region.
 10. A method formanufacturing a diode, comprising the steps of: preparing afirst-conductivity-type first region having a first impurityconcentration; placing a first-conductivity-type second region on saidfirst region, said second region having a second impurity concentrationthat is lower than said first impurity concentration, said first regionand said second region constituting a semiconductor substrate; forming agenerally ring-like second-conductivity-type third region in a surfacelayer of said second region; forming a first main electrode in contactwith said first region forming a third main electrode which contactswith a surface of said semiconductor substrate located inside said thirdregion, so as to form a Schottky junction; and forming afirst-conductivity-type fifth region in a surface layer of said secondregion located inside said second-conductivity-type third region, saidfifth region having a resistivity that is higher than that of saidsecond region, and having a thickness that is smaller than a diffusiondepth of said third region, wherein said fifth region is formed byimplanting second-conductivity-type ions in a dose amount of not largerthan 1×10¹³ cm⁻², and conducting heat treatment.
 11. A method accordingto claim 10, wherein said fifth region is formed by implanting thesecond-conductivity-type ions in a dose amount in a range of 1×10¹⁰ to1×10¹² cm⁻², and conducting heat treatment.
 12. A method formanufacturing a diode according to claim 10, wherein the heat treatmentis conducted at a temperature in a range of 300 to 600° C.
 13. A methodfor manufacturing a diode, comprising the steps of: preparing afirst-conductivity-type first region having a first impurityconcentration; placing a first-conductivity-type second region on saidfirst region, said second region having a second impurity concentrationthat is lower than said first impurity concentration, said first regionand said second region constituting a semiconductor substrate; forming agenerally ring-like second-conductivity-type third region in a surfacelayer of said second region; forming a first main electrode in contactwith said first region forming a third main electrode which contactswith a surface of said semiconductor substrate located inside said thirdregion, so as to form a Schottky junction; and forming afirst-conductivity-type fifth region in a surface layer of said secondregion located inside said second-conductivity-type third region, saidfifth region having a resistivity that is higher than that of saidsecond region, and having a thickness that is smaller than a diffusiondepth of said third region, wherein said fifth region is formed byepitaxial growth.
 14. A diode comprising: a first-conductivity-typefirst region having a first impurity concentration; afirst-conductivity-type second region placed on said first region andhaving a second impurity concentration that is lower than said firstimpurity concentration; a second-conductivity-type fourth region formedin a surface layer of said second region; a first-conductivity-typesixth region formed in contact with a portion of said first regionlocated below said fourth region, said sixth region having a lowerresistivity than said second region; a first main electrode formed incontact with said first region; and a second main electrode formed incontact with said fourth region; wherein said sixth region is formedonly inside an area defined by a vertical projection of said fourthregion.
 15. A diode comprising: a first-conductivity-type first regionhaving a first impurity concentration; a first-conductivity-type secondregion placed on said first region and having a second impurityconcentration that is lower than said first impurity concentration; agenerally ring-like second-conductivity-type third region formed in asurface layer of said second region; a second-conductivity-type fourthregion formed in a surface layer of said second region located insidesaid third region; a first-conductivity-type sixth region formed incontact with a portion of said first region that is located below saidfourth region surrounded by said third region, said sixth region havinga lower resistivity than said second region; a first main electrodeformed in contact with said first region; and a second main electrodeformed in contact with said fourth region, wherein said sixth region isformed only inside a vertical projection of said third region, and saidsecond region includes a first portion located on said sixth region, anda second portion located under said third region, said first portionhaving a smaller thickness than said second portion.
 16. A diodeaccording to claim 15, wherein said sixth region is formed inside thevertical projection of said third region, with a spacing of 5 μm orlarger between a periphery of the sixth region and the verticalprojection.
 17. A diode comprising: a first-conductivity-type firstregion having a first impurity concentration; a first-conductivity-typesecond region placed on said first region and having a second impurityconcentration that is lower than said first impurity concentration, saidfirst region and said second region constituting a semiconductorsubstrate; a generally ring-like second-conductivity-type third regionformed in a surface layer of said second region; afirst-conductivity-type sixth region formed in contact with a portion ofsaid first region that is located below a portion of said second regionthat is surrounded by said third region, said sixth region having alower resistivity than said second region; a first main electrode formedin contact with said first region; and a third main electrode whichcontacts with a surface of said semiconductor substrate located insidesaid third region, so as to form a Schottky junction, wherein saidsecond region includes a first portion located on said sixth region, anda second portion located under said third region, said first portionhaving a smaller thickness than said second portion.
 18. A diodeaccording to claim 17, wherein said sixth region is formed inside thevertical projection of said third region, with a spacing of 5 μm orlarger between a periphery of the sixth region and the verticalprojection.
 19. A diode comprising: a first-conductivity-type firstregion having a first impurity concentration; a first-conductivity-typesecond region placed on said first region and having a second impurityconcentration that is lower than said first impurity concentration; aplurality of trenches formed in a surface layer of said second region; asecond-conductivity-type fourth region formed along a surface of saidsecond region and inner faces of said trenches; afirst-conductivity-type sixth region formed in contact with said firstregion located below said trenches, said sixth region having a lowerresistivity than said second region; a first main electrode formed incontact with said first region; and a second main electrode formed incontact with said fourth region, wherein said sixth region is formedonly inside an area defined by a vertical projection of outermost onesof said trenches.
 20. A diode comprising: a first-conductivity-typefirst region having a first impurity concentration; afirst-conductivity-type second region placed on said first region andhaving a second impurity concentration that is lower than said firstimpurity concentration; a plurality of trenches formed in a surfacelayer of said second region; a first-conductivity-type sixth regionformed in contact with a portion of said first region located below saidtrenches, said sixth region having a lower resistivity than said secondregion; a first main electrode formed in contact with said first region;and a third main electrode formed in contact with at least a part of asurface of said second region and inner faces of said trenches, to forma Schottky junction, wherein said sixth region is formed only inside anarea defined by a vertical projection of outermost ones of saidtrenches.